The purpose of these experiments is to assess the ability of cellular oncogenes (c-onc) to transform cells in vivo. The transforming potential of two mouse c-onc genes, c-fos and c-mos, will be examined. A number of DNA constructs containing these c-onc genes in combination with dominant selectable marker genes, inducible promoters, and/or LTR enhancer regions will be made. These constructs will be transferred to mouse embryo-derived teratocarcinoma cells (EK cells) using DNA transfection procedures. Clonal lines derived from single transfected cells will be characterized to determine the number of inserted c-onc genes, the number of integration sites, and c-onc transcriptional activity. Transfected EK cells will be introduced into two in vivo test systems to assess the transforming potential of c-fos and c-mos. In the first system, transfected EK cells will be injected into mouse blastocysts to produce chimeric mice. The formation of tumors and c-onc transcriptional activity in these mice will be assessed. In the second system, transfected EK cells or chimeric embryos (blastocysts injected with transfected EK cells) will be transplanted to ectopic sites in adult mouse hosts and the resulting teratoid tumors will be characterized. The specific aims of these experiments are to determine whether: (1)\increased c-onc transcription is sufficient to induce tumor formation in a mouse and/or to alter the incidence of teratocarcinomas and teratomas formed as a result of transplanting embryos or teratocarcinoma cells to ectopic sites in adult hosts, or (2)\whether alterations in a c-onc gene are necessary to activate its transforming potential in vivo. Additionally, the results of these experiments will elucidate the effects of a number of factors, including genetic background, tissue specificity, and integration site(s), on the transforming potential of c-onc genes. An understanding of the transforming potential of these cellular genes will directly influence the adoption of strategies used in the treatment and, ultimately, in the prevention of human cancer. (X)